''We are seeing the beginning of a new era where the debate is no longer whether change is needed but how to achieve significant change,'' says Thomas Kochan, professor of industrial relations at the Massachusetts Institute of Technology in Cambridge, Mass. Ultimately, ''we will see a rekindling of spirit in the labor movement,'' Mr. Kochan says.

As two rival candidates grapple for the presidency of the AFL-CIO, an alliance of 80 unions with 13 million members, they are repudiating the complacency and closed governing style that helped steadily plunge private-sector union representation to the lowest level since the Great Depression.

During their contest this summer the candidates have converged toward a common outlook and strategy. The campaign platforms of the opponents - interim AFL-CIO president Thomas Donahue and service employee leader John Sweeney - are now nearly identical.

With the campaign, the federation is achieving ''a consensus to go forward and organize, to improve legislative activities, and to explain ourselves better to the rest of the country,'' Mr. Donahue said Wednesday after addressing a convention in Chicago of the Building and Construction Trades Department, an organization of 15 unions under the AFL-CIO.

Even with a unified leadership strategy, the list of obstacles to a dramatic turnaround in the labor movement is long. A mood of resignation is pervasive among workers. Many union members are cowed by decades of union malaise, wholesale layoffs, a more conservative Congress, shrinking real incomes, competition from low-cost foreign labor, eroding job benefits, and widespread employer opposition to unions.

But both Sweeney and Donahue pledge foremost to reverse the decline in blue-collar incomes by launching an intensive national campaign to unionize workers. After years of little or no grass-roots recruitment by the AFL-CIO for its affiliated unions, the candidates promise to devote millions of dollars and hundreds of specialists to the task.

The widely hailed strategic shift would move the federation away from its failed focus on both lobbying in Washington and sustaining the benefits and wages of union members, or the status quo.

By reaching out boldly to non-union labor, the candidates hope to stem the decline of what was once one of the nation's most potent social movements. (Union representation in the private sector has sunk from a peak of 36 percent in 1953 to about 11 percent today.) A revivified AFL-CIO would be better able to combat efforts by conservative lawmakers to pass legislation that labor opposes.

Moreover, the candidates, who are both white, pledge to open up top federation posts to women and minorities. White men dominate the AFL-CIO even though 3 out of every 5 workers who accept a union card are women or members of a minority.

Although the candidates share much common ground (and often refer to their longstanding friendship), the contest has crackled with tension as each has questioned the other's credibility.

Mr. Sweeney, president of the Service Employees International Union (SEIU), says Mr. Donahue has copied his campaign platform point-by-point. He asks how Donahue can promote himself as the best choice for advancing the most critical federation task - labor organizing - when Sweeney has logged the best recruitment record among leaders of the affiliated unions. (Sweeney has expanded the roll of his union from 626,000 members in 1980 to more than 1.1 million members today.)

For his part, Donahue quotes statements by Sweeney in April and May strongly supporting Donahue as the best choice for federation president.

Later, Sweeney withdrew his support from Donahue and sought to lead the federation following a failed attempt to oust former AFL-CIO President Lane Kirkland in favor of Donahue. Mr. Kirkland refused to step down and Donahue, then the No. 2 man at the federation, said he would remain loyal to Kirkland and not seek the top post but retire.

Kirkland's announcement in June that he would indeed retire opened the way for Donahue to seek the post of federation president, but in opposition to Sweeney.

Donahue ''can't understand why we are not accepting the fact that he sent out a letter telling everybody he retired and then changed his mind,'' Sweeney says.

''The train [of reform] left without him,'' Sweeney adds. Sweeney's backers say that he has the support of 25 unions and 60 percent of the votes to be cast in the October election of the new president.

Judging from the reaction of some of the 237 delegates at the Building Trades convention on Wednesday, workers are largely ignoring the campaign squabbling and seizing on the prospect of a more dynamic federation.

''The idea that we've all been given is that labor is headed down a bad spiral, but I believe we'll see a big resurgence in unions in coming years,'' says Eric Andersen, a roofer from Eau Claire, Wis.

On the other side of the bargaining table, the stirrings of labor have prompted US executives toward vigilance but not alarm, says Michael Baroody, vice president of public affairs at the National Association of Manufacturers in Washington.

The campaign ''is clearly a help in stimulating interest in the movement and stimulating an understanding among all of our membership for the need for growth and the need for organizing,'' Donahue says. But adds the race has taxed the candidates' camaraderie: ''The campaign is putting a real strain on our friendship''.